Collector for EUV light source

ABSTRACT

An apparatus/method may comprise, a multi-layer reflecting coating forming an EUV reflective surface which may comprise an inter-diffusion barrier layer which may comprise a carbide selected from the group ZrC and NbC or a boride selected from the group ZrB 2  and NbB 2  or a disilicide selected from the group ZrSi 2  and NbSi 2  or a nitride selected from the group BN, ZrN, NbN, BN, ScN and Si 3 N 4 . The apparatus and method may comprise an EUV light source collector which may comprise a collecting mirror which may comprise a normal angle of incidence multi-layer reflecting coating; an inter-diffusion barrier layer comprising a material selected from the group comprising a carbide selected from the group ZrC and NbC, or a boride selected from the group ZrB 2  and NbB 2  or a disilicide selected from the group ZrSi 2  and NbSi 2  a nitride selected from the group BN, ZrN, NbN, BN, ScN and Si 3 N 4 .

RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 10/798,740, filed Mar.10, 2004, which is a continuation-in-part of U.S. Ser. No. 10/409,254filed Apr. 8, 2003, the disclosure of which is incorporated by referenceherein.

FIELD

The disclosed subject matter relates to the field of the generation ofEUV (soft-x-ray) light for such applications as semiconductor integratedcircuit lithography exposure light sources, and more particularly tolight collectors for such devices.

BACKGROUND

The need for such applications as ever increasingly smaller criticaldimensions for semiconductor integrated circuit manufacturing the needhas arisen to move from the generation of Deep Ultraviolet (“DUV”) lightto Extreme Ultraviolet (“EUV”) light, also referred to as soft-x-raylight. Various proposals exist for apparatus and methods for thegeneration of such light at effective energy levels to enable, e.g.,adequate throughput in an EUV lithography tool (e.g., a stepper scanneror scanner) over an acceptable lifetime between, e.g., replacements ofmajor components.

Proposals exist for generating, e.g., light centered at a wavelength of13.5 nm using, e.g., Lithium which is introduced into and/or irradiatedto form a plasma which excites the lithium atoms to states from whichdecay results in large part in EUV light photons having an energydistribution centered about 13.5 nm. The plasma may be formed by anelectrical discharge using a dense plasma focus electrode in thevicinity of a source of lithium in solid or liquid form, e.g., asdiscussed in U.S. Pat. No. 6,586,757, entitled PLASMA FOCUS LIGHT SOURCEWITH ACTIVE BUFFER GAS CONTROL, issued to Melynchuk et al. on Jul. 1,2003, and the above referenced patent application Ser. No. 10/409,254filed Apr. 8, 2003, and U.S. Pat. No. 6,566,668, entitled PLASMA FOCUSLIGHT SOURCE WITH TANDEM ELLIPSOIDAL MIRROR UNITS, issued to Rauch etal. on May 20, 2003, and U.S. Pat. No. 6,566,667, entitled PLASMA FOCUSLIGHT SOURCE WITH IMPROVED PULSE POWER SYSTEM, issued to Partlo et al onMay 20, 2003, which are assigned to the assignee of the presentapplication and applications and patents and other references referencedtherein, the disclosures of all of which are hereby incorporated byreference, and also other representative patents or publishedapplications, e.g., United States Published application No.2002-0009176A1, entitled X-RAY EXPOSURE APPARATUS, published on Jan. 24,2002, with inventors Amemlya et al. the disclosures of which are herebyincorporated by reference. In addition, as noted in, e.g., patents andpublished applications U.S. Pat. No. 6,285,743, entitled METHOD ANDAPPARATUS FOR SOFT X-RAY GENERATION, issued to Kondo et al. on Sep. 4,2001, U.S. Pat. No. 6,493,423, entitled METHOD OF GENERATING EXTREMELYSHORT-WAVE RADIATION . . . , issued to Bisschops on Dec. 10, 2002,United States Published application 2002-0141536A1 entitled EUV, XUV ANDX-RAY WAVELENGTH SOURCES CREATED FROM LASER PLASMA Published on Oct. 3,2002, with inventor Richardson, U.S. Pat. No. 6,377,651, entitled LASERPLASMA SOURCE FOR EXTREME ULTRAVIOLET LITHOGRAPHY USING WATER DROPLETTARGET, issued to Richardson et al. on Apr. 23, 2002, U.S. Pat. No.6,307,913, entitled SHAPED SOURCE OF X-RAY, EXTREME ULTRAVIOLET ANDULTRAVIOLET RADIATION, issued to Foster et al. on Oct. 23, 2001, thedisclosures of which are hereby incorporated by reference, the plasmamay be induced by irradiating a target, e.g., a droplet of liquid metal,e.g., lithium or a droplet of other material containing a target of, ametal, e.g., lithium within the droplet, in liquid or solid form, with,e.g., a laser focused on the target.

Since the amount of energy in the EUV light desired to be producedwithin the desired bandwidth, from the creation of such a plasma andresultant generation from the plasma of EUV light, is relativelyenormous, e.g., 100 Watts/cm², its is necessary to ensure that theefficiency of the collection of the EUV light be made as high aspossible. It is also required that this efficiency not significantlydeteriorate, i.e., be able to sustain such high efficiency, overrelatively extended periods of operation, e.g., effectively a year ofoperation at very high pulse repetition rates (4 KHz and above) for aneffective 100% duty cycle. Many challenges exist to being able to meetthese goals aspects of which are dealt with in explaining aspects of thedisclosed subject matter regarding a collector for an EUV light source.

Some issues that are required to be addressed in a workable designinclude, e.g., Li diffusion into the layers of a multi layer normalangle of incidence reflecting mirror, e.g., through an outer coating ofruthenium (“Ru”), with the multilayered mirror made, e.g., ofalternating layers of Molybdenum (“Mo” or “Moly”) and silicon (“Si”) andthe impact on, e.g., the primary and/or secondary collector lifetime;chemical reactions between, e.g., Li and Si and the impact on, e.g., theprimary and/or secondary collector lifetimes; scatter of out of bandradiation, e.g., from the laser producing the irradiation for ignitionto form the plasma, e.g., 248 nm radiation from an KrF excimer laserrequired to be kept low to avoid any impact on resist exposure giventhat Deep UV resist types may be carried over into the EUV range oflithography and such out of band light scattered from the target canresult in exposing the resist very efficiently; achieving a 100 Wdelivery of output light energy to the intermediate focus; having alifetime of a primary and secondary collector of at least 5 G pulses;achieving the required conversion efficiency with a given source, e.g.,a given target, e.g., a target droplet or target within a droplet, orother targets, the preservation of lifetime of the required multi layermirrors at operational elevated temperatures and out of band radiationat center wavelengths near, e.g., 13.5 nm.

It is well known that that normal incidence of reflection (“NIR”)mirrors can be constructed for wavelengths of interest in EUV, e.g.,between about 5 and 20 nm, e.g., around 11.3 nm or 13.0-13.5 nmutilizing multi-layer reflection. The properties of such mirrors dependupon the composition, number, order, crystallinity, surface roughness,interdiffusion, period and thickness ratio, amount of annealing and thelike for some or all of the layers involved and also, e.g., such thingsas whether or not diffusion barriers are used and what the material andthickness of the barrier layer is and its impact on the composition ofthe layers separated by the barrier layer, as discussed, e.g., in Braun,et al., “Multi-component EUV multi-layer mirrors, Proc. SPIE 5037 (2003)(Braun”); Feigl, et al., “Heat resistance of EUV multi-layer mirrors forlong-time applications,” Microelectronic Engineering 57-58, p. 3-8(2001) (“Feigl”), U.S. Pat. No. 6,396,900, entitled MULTILAYER FILMSWITH SHARP, STABLE INTERFACES FOR USE IN EUV AND SOFT X-RAY APPLICATION,issued to Barbee, Jr. et al. on May 28, 2002, based upon an applicationSer. No. 10/847,744, filed on May 1, 2002 (“Barbee”) and U.S. Pat. No.5,319,695, entitled MULTILAYER FILM REFLECTOR FOR SOFT X-RAYS, issued toItoh et al. on Jun. 7, 1994, based on an Application Ser. No. 45,763,filed on Apr. 14, 1993, claiming priority to a Japanese applicationfiled on Apr. 21, 1992 (“Itoh”).

Itoh discusses materials of different X-ray refractive indexes, forexample, silicon (Si) and molybdenum (Mo), alternately deposited on asubstrate to form a multilayer film composed of silicon and molybdenumlayers and a hydrogenated interface layer formed between each pair ofadjacent layers. Barbee discusses a thin layer of a third compound,e.g., boron carbide (B₄C), placed on both interfaces (Mo-on-Si andSi-on-Mo interface). This third layer comprises boron carbide and othercarbon and boron based compounds characterized as having a lowabsorption in EUV wavelengths and soft X-ray wavelengths. Thus, amulti-layer film comprising alternating layers of Mb and Si includes athin interlayer of boron carbide (e.g., B₄C) and/or boron basedcompounds between each layer. The interlayer changes the surface(interface) chemistry, which can result in an increase of thereflectance and increased thermal stability, e.g., for Mo/Si whereinter-diffusion may be prevented or reduced, resulting in these desiredeffects. Barbee also discusses varying the thickness of the third layerfrom the Mo-on-Si interface to the Si-on-Mo interface. Barbee alsodiscusses the fact that typically the sharpness of the Mo-on-Siinterface would be about 2.5 times worse than that of the Si-on-Mointerface; however, due to the deposition of the interlayer of B₄C inthe Mo-on-Si interface, such interface sharpness is comparable to thatof the Si-on-Mo interface. Braun discusses the use of carbon barrierlayers to reduce inter-diffusion at the Mo—Si boundaries to improve thethermal stability and lower internal stress and at the same timeincreasing reflectivity. Braun notes that normally the Mo—Si boundaryforms MoSi₂ at the interface in varying thicknesses at the Mo-on-Siboundary and the Si-on-Mo boundary, and also that the morphology of theMo and/or Si layers can be influenced by barrier layers of, e.g., carboncontent. In addition, Braun notes the impact of barrier layer formationon interface roughness of the Mo—Si interface without a barrier layer.Braun reports a reflectance at λ=13.3 mm of 70.1% using Mo/SiCmulti-layers. The reduction in internal stress using B₄C even withannealing as compared to Mo/Si/C multi-layers, which impacts the abilityto uses such multi-layer mirrors for curved mirrors is also discussed.Braun also discusses the tradeoff between interlayer contrast, impactingreflectivity, and absorption in the multi-layer configurations, suchthat, e.g., NbSi layers with lower absorption in the Nb but also lowercontrast, and Ru/Si with higher contrast but also higher absorption inthe Ru layer, both performing less effectively than a Mo/Si multi-layerstack. Braun also discusses the theoretical utility of using threelayers of, e.g., Mo/Si/Ag or Mo/Si/Ru, which have theoretically higherreflectivity, but that the Ag embodiment fails to achieve thetheoretical reflectivity due to voids in the Ag layer at desiredthicknesses and a calculated best reflectivity of a Mo/Si/C/Rumulti-layer stack at λ=13.5 nm, with a thickness constrained in the Molayer to prevent crystallization in the Mo layer. However, Braun alsofinds that the Mo/Si/C/Ru multi-layer stacks do not live up totheoretical calculated reflectivity expectation, probably due to aninitial Mo layer deposition surface roughness that propagates upwardthrough the stack. Feigl discusses the impact of elevated temperaturesup to 500° C. on the structural stability of, e.g., Mo/Si andMo/Mo₂C/Si/Mo₂C multilayer stacks, including the use of ultra thin Mo₂Cbarrier layers. Feigl notes that the barrier layer prevents theformation of inter-diffusion layers of MoSi_(x) due to annealing of theMo and Si at temperatures above, e.g., 200° C. and that Mo/Mo₂C/Si/Mo₂Cand Mo₂C/Si systems remain stable up to 600° C. The former system havingultra thin Mo₂C barrier layers (MoSi₂ is also suggested but not tested)layers and the latter is formed by substituting Mo₂C for Mo in amultilayer system. The reflectivity of the Mo₂C/Si system remained above0.8 through 600° C. according to Feigl, whereas the Mo/Mo₂C/Si/Mo₂Csystem tailed off to slightly less than 0.7 at that temperature, andeven decreased to about 0.7 at 400° C.

Applicants in the present application propose certain other materialsfor barrier layers and other potential improvements to the multi-layerstack for EUV applications.

SUMMARY

It will be understood by those skilled in the art that an apparatus andmethod is disclosed which may comprise, a multi-layer reflecting coatingforming an EUV reflective surface which may comprise an inter-diffusionbarrier layer which may comprise a carbide selected from the group ZrCand NbC or a boride selected from the group ZrB₂ and NbB₂ or adisilicide selected from the group ZrSi₂ and NbSi₂ or a nitride selectedfrom the group BN, ZrN, NbN, BN, ScN and Si₃N₄. The apparatus and methodmay comprise an EUV light source collector which may comprise acollecting mirror which may comprise a normal angle of incidencemulti-layer reflecting coating; an inter-diffusion barrier layercomprising a material selected from the group comprising a carbideselected from the group ZrC and NbC, or a boride selected from the groupZrB₂ and NbB₂ or a disilicide selected from the group ZrSi₂ and NbSi₂ anitride selected from the group BN, ZrN, NbN, BN, ScN and Si₃N₄. Themulti-layer reflecting coating may comprise a capping layer comprisingruthenium. The multilayer reflecting coating may comprise multiplealternating layers of an absorber and a separator. The absorber maycomprise molybdenum and the separator may comprise silicon. The cappinglayer may comprise molybdenum. The sputter thickness rate for sputteringof the capping layer material by material comprising a plasma productionmaterial may be at or below a rate that will result in the capping layermaterial sustaining such sputtering for greater than a selectedlifetime. The multi-layer reflecting coating may comprise a cappinglayer which may comprise a material other than the absorber material andthe separator material selected to have a sputter thickness rate thatwill sustain sputtering by a material comprising a plasma productionmaterial at or below a rate that will result in a single layer of thecapping layer material sustaining such sputtering for greater than aselected time and to have more favorable properties when exposed toambient or operating environments than those of the absorber material orthe separator material. The multilayer reflecting coating may comprisemultiple alternating layers which may comprise a multi-layer mirroroptimized for a selected nominal center wavelength in the EUV range. Theapparatus and method may comprise a multi-layer reflecting coatingforming an EUV reflective surface which may comprise an inter-diffusionbarrier layer comprising yttrium, scandium or strontium. The apparatusand method may comprise an EUV light source collector which may comprisea collecting mirror which may comprise a normal angle of incidencemulti-layer reflecting coating; an inter-diffusion barrier layercomprising a material comprising yttrium, scandium or strontium. Themultilayer reflecting coating may comprise multiple alternating layerswhich may comprise MoSi₂/Si or Mo₂C/Si, with or without aninter-diffusion barrier layer or inter-diffusion barrier layers. Themultilayer reflecting coating may comprise multiple alternating layerswhich may comprise Mo/X/Si/X where X may comprise an inter-diffusionbarrier layer which may comprise C or a carbide selected from the groupZrC and NbC, or a boride selected from the group ZrB₂ and NbB₂, or adisilicide selected from the group ZrSi₂ and NbSi₂ or a nitride selectedfrom the group BN, ZrN, NbN, BN, ScN and Si₃N₄.

A method and apparatus for debris removal from a reflecting surface ofan EUV collector in an EUV light source is disclosed which may comprisethe reflecting surface comprises a first material and the debriscomprises a second material and/or compounds of the second material, thesystem and method may comprise a controlled sputtering ion source whichmay comprise a gas comprising the atoms of the sputtering ion material;and a stimulating mechanism exciting the atoms of the sputtering ionmaterial into an ionized state, the ionized state being selected to havea distribution around a selected energy peak that has a high probabilityof sputtering the second material and a very low probability ofsputtering the first material. The stimulating mechanism may comprise anRF or microwave induction mechanism. The gas is maintained at a pressurethat in part determines the selected energy peak and the stimulatingmechanism may create an influx of ions of the sputtering ion materialthat creates a sputter density of atoms of the second material from thereflector surface that equals or exceeds the influx rate of the plasmadebris atoms of the second material. A sputtering rate may be selectedfor a given desired life of the reflecting surface. The reflectingsurface may be capped. The collector may comprise an elliptical mirrorand a debris shield which may comprise radially extending channels. Thefirst material may be molybdenum, the second lithium and the ionmaterial may be helium. The system may have a heater to evaporate thesecond material from the reflecting surface. The stimulating mechanismmay be connected to the reflecting surface between ignition times. Thereflecting surface may have barrier layers. The collector may be aspherical mirror in combination with grazing angle of incidencereflector shells, which may act as a spectral filter by selection of thelayer material for multi-layer stacks on the reflector shells. Thesputtering can be in combination with heating, the latter removing thelithium and the former removing compounds of lithium, and the sputteringmay be by ions produced in the plasma rather than excited gas atoms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an overall broad conception for alaser-produced plasma EUV light source according to an aspect of thedisclosed subject matter;

FIG. 1A shows schematically the operation of the system controlleraccording to an aspect of an embodiment of the disclosed subject matter;

FIG. 2A shows a side view of an embodiment of an EUV light collectoraccording to an aspect of the disclosed subject matter looking from anirradiation ignition point toward an embodiment of a collector accordingto an embodiment of the disclosed subject matter;

FIG. 2B shows a cross-sectional view of the embodiment of FIG. 2A alongthe lines 2B in FIG. 2A;

FIG. 3 shows an alternative embodiment of a normal angle of incidencecollector according to an aspect of the disclosed subject matter;

FIG. 4 shows a schematic view of a normal angle of incidence collectordebris management system according to an aspect of the disclosed subjectmatter;

FIGS. 5 a-c show a timing of the provision of a collector cleaningsignal/current at RF and/or DC to the collector mirror according to anaspect of an embodiment of the disclosed subject matter;

FIGS. 6 a and b show schematic views in cross section of aspects ofembodiments of the disclosed subject matter relating to grazing angle ofincidence collectors;

FIG. 7 shows a plot of grazing angle of incidence reflectivity for avariety of reflective surfaces at given wavelengths of relevance at anangle of incidence of 5 degrees;

FIG. 8 shows a plot of grazing angles of incidence reflectivity for avariety of reflective surfaces at given wavelengths of relevance for 15degrees;

FIG. 9 shows a schematic view of an alternative embodiment of acollector according to an aspect of the disclosed subject matter;

FIG. 10 shows a calculated number of lithium atoms per droplet vs.droplet diameter, useful in illustrating an aspect of an embodiment ofthe disclosed subject matter;

FIG. 11 shows a calculated influx of lithium atoms onto a mirror surfacevs. mirror radius useful in illustrating an aspect of an embodiment ofthe disclosed subject matter;

FIG. 12 shows a calculated required lithium thickness sputter rate vs.mirror diameter useful in illustrating an aspect of an embodiment of thedisclosed subject matter.

FIG. 13 shows a required ratio of molybdenum sputter rate to lithiumsputter rate vs. mirror radius in order to have a 1-year life with a 300pair multi-layer coated mirror useful in illustrating an aspect of anembodiment of the disclosed subject matter;

FIG. 14 shows sputter yield for lithium, silicon, and molybdenum withhelium ions useful in illustrating an aspect of an embodiment of thedisclosed subject matter;

FIG. 15 shows normalized helium ion energy along with sputter yields forlithium, silicon, and molybdenum useful in illustrating an aspect of anembodiment of the disclosed subject matter;

FIG. 16 shows helium ion current density along with the sputter yield oflithium, silicon, and molybdenum useful in illustrating an aspect of anembodiment of the disclosed subject matter;

FIG. 17 shows total helium ion sputter rates for lithium, silicon, andmolybdenum useful in illustrating an aspect of an embodiment of thedisclosed subject matter;

FIG. 18 shows normalized lithium ion energy along with sputter yieldsfor lithium and molybdenum useful in illustrating an aspect of anembodiment of the disclosed subject matter;

FIG. 19 shows a radiated power density vs. temperature for a black bodyuseful in illustrating an aspect of an embodiment of the disclosedsubject matter;

FIG. 20 shows a schematic view of an aspect of an embodiment of thedisclosed subject matter;

FIGS. 21 A and B show results of experiments regarding the stoppingpower of helium and argon buffer gases against both tin and lithium ionsaccording to aspects of an embodiment of the disclosed subject matter;and,

FIGS. 22A-E show results of further examination of the stopping power ofhelium and argon buffer gases against both lithium and tin according toaspects of an embodiment of the disclosed subject matter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to FIG. 1 there is shown a schematic view of an overallbroad conception for an EUV light source, e.g., a laser produced plasmaEUV light source 20 according to an aspect of the disclosed subjectmatter. The light source 20 may contain a pulsed laser system 22, e.g.,a gas discharge laser, e.g., an excimer gas discharge laser, e.g., a KrFor ArF laser operating at high power and high pulse repetition rate andmay be a MOPA configured laser system, e.g., as shown in U.S. Pat. Nos.6,625,191, 6,549,551, and 6,567,450. The laser may also be, e.g., asolid state laser, e.g., a YAG laser. The light source 20 may alsoinclude a target delivery system 24, e.g., delivering targets in theform of liquid droplets, solid particles or solid particles containedwithin liquid droplets. The targets may be delivered by the targetdelivery system 24, e.g., into the interior of a chamber 26 to anirradiation site 28, otherwise known as an ignition site or the sight ofthe fire ball. Embodiments of the target delivery system 24 aredescribed in more detail below.

Laser pulses delivered from the pulsed laser system 22 along a laseroptical axis 55 through a window (not shown) in the chamber 26 to theirradiation site, suitably focused, as discussed in more detail below incoordination with the arrival of a target produced by the targetdelivery system 24 to create an ignition or fire ball that forms anx-ray (or soft x-ray (EUV) releasing plasma, having certaincharacteristics, including wavelength of the x-ray light produced, typeand amount of debris released from the plasma during or after ignition,according to the material of the target.

The light source may also include a collector 30. e.g., a reflector,e.g., in the form of a truncated ellipse, with an aperture for the laserlight to enter to the ignition site 28. Embodiments of the collectorsystem are described in more detail below. The collector 30 may be,e.g., an elliptical mirror that has a first focus at the ignition site28 and a second focus at the so-called intermediate point 40 (alsocalled the intermediate focus 40) where the EUV light is output from thelight source and input to, e.g., an integrated circuit lithography tool(not shown). The system 20 may also include a target position detectionsystem 42. The pulsed system 22 may include, e.g., a masteroscillator-power amplifier (“MOPA”) configured dual chambered gasdischarge laser system having, e.g., an oscillator laser system 44 andan amplifier laser system 48, with, e.g., a magnetic reactor-switchedpulse compression and timing circuit 50 for the oscillator laser system44 and a magnetic reactor-switched pulse compression and timing circuit52 for the amplifier laser system 48, along with a pulse power timingmonitoring system 54 for the oscillator laser system 44 and a pulsepower timing monitoring system 56 for the amplifier laser system 48. Thepulse power system may include power for creating laser output from,e.g., a YAG laser. The system 20 may also include an EUV light sourcecontroller system 60, which may also include, e.g., a target positiondetection feedback system 62 and a firing control system 65, along with,e.g., a laser beam positioning system 66.

The target position detection system may include a plurality of dropletimagers 70, 72 and 74 that provide input relative to the position of atarget droplet, e.g., relative to the ignition site and provide theseinputs to the target position detection feedback system, which can,e.g., compute a target position and trajectory, from which a targeterror cam be computed, if not on a droplet by droplet basis then onaverage, which is then provided as an input to the system controller 60,which can, e.g., provide a laser position and direction correctionsignal, e.g., to the laser beam positioning system 66 that the laserbeam positioning system can use, e.g., to control the position anddirection of the laser position and direction changer 68, e.g., tochange the focus point of the laser beam to a different ignition point28.

The imager 72 may, e.g., be aimed along an imaging line 75, e.g.,aligned with a desired trajectory path of a target droplet 94 from thetarget delivery mechanism 92 to the desired ignition site 28 and theimagers 74 and 76 may, e.g., be aimed along intersecting imaging lines76 and 78 that intersect, e.g., alone the desired trajectory path atsome point 80 along the path before the desired ignition site 28.

The target delivery control system 90, in response to a signal from thesystem controller 60 may, e.g., modify the release point of the targetdroplets 94 as released by the target delivery mechanism 92 to correctfor errors in the target droplets arriving at the desired ignition site28.

An EUV light source detector 100 at or near the intermediate focus 40may also provide feedback to the system controller 60 that can be, e.g.,indicative of the errors in such things as the timing and focus of thelaser pulses to properly intercept the target droplets in the rightplace and time for effective and efficient LPP EUV light production.

Turning now to FIG. 1A there is shown schematically further details of acontroller system 60 and the associated monitoring and control systems,62, 64 and 66 as shown in FIG. 1. The controller may receive, e.g., aplurality of position signal 134, 136 a trajectory signal 136 from thetarget position detection feedback system, e.g., correlated to a systemclock signal provided by a system clock 116 to the system componentsover a clock bus 115. The controller 60 may have a pre-arrival trackingand timing system 110 which can, e.g., compute the actual position ofthe target at some point in system time and a target trajectorycomputation system 112, which can, e.g., compute the actual trajectoryof a target drop at some system time, and an irradiation site temporaland spatial error computation system 114, that can, e.g., compute atemporal and a spatial error signal compared to some desired point inspace and time for ignition to occur.

The controller 60 may then, e.g., provide the temporal error signal 140to the firing control system 64 and the spatial error signal 138 to thelaser beam positioning system 66. The firing control system may computeand provide to a resonance charger portion 118 of the oscillator laser44 magnetic reactor-switched pulse compression and timing circuit 50 aresonant charger initiation signal 122 and may provide, e.g., to aresonance charger portion 120 of the PA magnetic reactor-switched pulsecompression and timing circuit 52 a resonant charger initiation signal,which may both be the same signal, and may provide to a compressioncircuit portion 126 of the oscillator laser 44 magnetic reactor-switchedpulse compression and timing circuit 50 a trigger signal 130 and to acompression circuit portion 128 of the amplifier laser system 48magnetic reactor-switched pulse compression and timing circuit 52 atrigger signal 132, which may not be the same signal and may be computedin part from the temporal error signal 140 and from inputs from thelight out detection apparatus 54 and 56, respectively for the oscillatorlaser system and the amplifier laser system.

The spatial error signal may be provided to the laser beam position anddirection control system 66, which may provide, e.g., a firing pointsignal and a line of sight signal to the laser bean positioner whichmay, e.g. position the laser to change the focus point for the ignitionsite 28 by changing either or both of the position of the output of thelaser system amplifier laser 48 at time of fire and the aiming directionof the laser output beam.

Turning now to FIGS. 2A and 2B there is shown, respectively a schematicview side view of a collector 30 looking into the collector mirror 150,and a cross-sectional view of the rotationally symmetric collectormirror 150 arrangement along cross-sectional lines 2B in FIG. 2A(although the cross-sectional view would be the same along any radialaxis in FIG. 2A.

As shown in FIG. 2A the elliptical collection mirror 150 is circular incross section looking at the mirror, which may be the cross-section atthe greatest extension of the mirror, which is shown in FIG. 1A to bealmost to the focus point 28 of the elliptical mirror 150, so as not toblock target droplets 94 from reaching the ignition point designed to beat the focus point 28. It will be understood, however, that the mirrormay extend further towards the intermediate focus, with a suitable holein the mirror (not shown) to allow passage of the target droplets to thefocus point. The elliptical mirror may also have an aperture 152, e.g.,shown to be circular in FIG. 2A, to allow entry of the LPP laser beam154, e.g., focused through focusing optics 156, through the mirror 150to the ignition point 28 desired to be at the focus of the ellipticalmirror. The aperture 152 can also be, e.g., more tailored to the beamprofile, e.g., generally rectangular, within the requirements, if any ofmodifying the beam optical path to make corrections of the focus of thelaser beam 154 on an ignition site, depending upon the type of controlsystem employed.

Also shown in FIGS. 2A and 2B is a debris shield 180 according to anaspect of an embodiment of the disclosed subject matter. The debrisshield 180 may be made up of a plurality of thin plates 182, made, e.g.,of thin foils of molybdenum, extending radially outward from the desiredignition site and defining narrow planar radially extending channels 184through the debris shield 180. The illustration of FIG. 2A is veryschematic and not to scale and in reality the channels are as thin ascan possibly be made. Preferably the foil plates 182 can be made to beeven thinner than the channels 184, to block as little of the x-raylight emitted from the plasma formed by ignition of a target droplet 94by the laser beam 155 focused on the ignition site 28.

Seen in cross section in FIG. 2B, the functioning of the channels 182 inthe debris shield 180 can be seen. A single radial channel is seen inFIG. 2B and the same would be seen in any section of the collector 30through the rotationally symmetric axis of rotation of the collectormirror 150 and debris shield 180 within a channel of the debris shield180. Each ray 190 of EUV light (and other light energy) emitted from theignition site 28 traveling radially outward from the ignition site 28will pass through a respective channel 182 in the debris shield 180,which as shown in FIG. 2B may, if desired, extend all the way to thecollection mirror 150 reflective surface. Upon striking the surface ofthe elliptical mirror 150, at any angle of incidence, the ray 190 willbe reflected back within the same channel 180 as a reflected ray 192focused on the intermediate focus 40 shown in FIG. 1.

Turning now to FIG. 3 there is shown an alternative embodiment accordingto an aspect of an embodiment of the disclosed subject matter. In thisembodiment, the debris shield 180 is not shown for simplicity and thisembodiment can be utilized with or without a debris shield asappropriate, as discussed in more detail below, as can also, e.g., thesingle elliptical collector mirror shown in FIGS. 2A and B. In thisembodiment a secondary collector reflecting mirror 200 has been added,which may comprise, e.g., a section of a spherical mirror 202, having acenter at the ignition site 28, i.e., the focus of the elliptical mirror150, and with an aperture 210 for the passage of the light from thecollector mirror 150 to the intermediate focus 40 (shown in FIG. 1). Thecollector mirror 150 functions as discussed above in regard to FIGS. 2Aand 2B with respect to rays 190 emitted from the ignition point 28toward the collector mirror 150. Rays of light 204 emitted from theignition site 28 away from the collector mirror 150 which strike thesection of the spherical mirror 202, will be reflected back through thefocus of the elliptical collector mirror 132 and, pass on to theelliptical collector mirror 150 as if emitted from the focus 28 of theelliptical mirror 150, and, therefore also be focused to theintermediate focus 40. It will be apparent that this will occur with orwithout the presence of the debris shield 180 as described in relationto FIGS. 2A and 2B.

Turning now to FIG. 4 there is shown schematically another aspect ofdebris management according to an embodiment of the disclosed subjectmatter. FIG. 4 shows a collector mirror 150 connected to a source ofcurrent, e.g., DC voltage source 220. This current can be, e.g., oneembodiment of the disclosed subject matter in which the currentmaintains the reflector at a selected temperature to, e.g., evaporatedeposited lithium. An alternate concept for lithium removal from thefirst collector mirror is to employ helium ion or hydrogen ionsputtering. The low mass of these ions, when kept at low energies (<50eV), e.g., can lead to extremely low sputter yield for, e.g., themolybdenum layer and/or the silicon layer, e.g., in an EUV multi-layermirror fabricated with Mo/Si layers.

Turning now to FIG. 4 there is shown a debris cleaning arrangementaccording to an aspect of an embodiment of the disclosed subject matter.As shown in FIG. 4, a source of current, e.g., DC voltage source 220may, e.g., be connected to the collector mirror 150, e.g., to a metal,e.g., aluminum or nickel backing (not shown) for the mirror 150. Themirror 150 may thus be heated to an elevated temperature above that ofthe surrounding gas, e.g., helium gas, making up the content of the EUVlight source chamber 26 interior. Other heating of the reflector mayoccur according to alternative embodiments of the disclosed subjectmatter, e.g., by radiant heating from, e.g., a heat lamp (not shown) inthe vessel 26.

Another aspect of debris cleaning may incorporate, e.g., as shown inFIG. 4, e.g., the introduction of RF, e.g., from a source of RFfrequency voltage 230 and an antenna, shown schematically at 232 withinthe chamber 26 in FIG. 4. In fact, the RF, as with the DC shown in FIG.4, may be connected to the mirror 150 or a metallic backing (not shown)in which event a dark shield (not shown) made of a suitable conductivematerial and connected to ground potential may be formed over the backof the collector mirror 150, separated from the mirror 150 by aninsulator, e.g., an air gap, and the potential, e.g., DC from DC source220 that is connected also to the mirror 150.

As shown in FIGS. 5 a-c, for a given periodic LPP ignition at times t₁,t₂, t₃, the RF may be replaced by a DC voltage during the time when theignitions occur at t₁, t₂, t₃, and for a short time on either side ofthe ignition time, with RF between such times, at least directly afterthe ignition, if not completely through the next occurrence of the DCpotential during the next ignition. Also shown is that the DC fromsource 220 may be a positive potential during the time of the respectiveignition, perhaps coextensive with the continuous voltage from the RFsource 230, and a negative potential between such positive pulses.

The voltage applied to the collector mirror 150 is meant to, on the onehand, evaporate metallic debris, e.g., lithium emitted from the plasmaduring and after ignition of a target droplet of such lithium or othertarget metallic material. Also evaporated could be metallic elementssuch as K, Fe, Na or the like that appear due, e.g., to impurities inthe lithium target droplets themselves and are similarly deposited onthe collector mirror 150 surface after ignition.

The RF is meant to form a localized ionic plasma, e.g., of excited Heatoms in the vicinity of the collector mirror 150 surface, with theintent that these excited ions in the localized plasma may strikelithium atoms or compounds of lithium on the collector mirror 150 andsputter them off of the mirror surface. This embodiment of the disclosedsubject matter contemplates, e.g., a balancing between the evaporationmechanism and the sputtering mechanism, e.g., if the RF is at <500 Wpower (at 13.65 MHz, as dictated by federal regulations for RF frequencysputtering) then the mirror temperature should be maintained at or nearsome desired temperature and if the RF is increased, e.g., to >500 W at13.65 MHz then the temperature can correspondingly be reduced.

Turning now to FIGS. 6A and B there are shown aspects of embodiments ofthe disclosed subject matter relating to alternative collectorarrangements. As shown in FIGS. 6A and B a collector 225 may be composedof, e.g., a plurality of nested shells, forming, e.g., differentsections made up, e.g., of elliptical and parabolic reflecting shells,e.g., in FIG. 6 a parabolic shells 230 and 240 and elliptical shells 250and 260. The elliptical shells, e.g., 230 and 240 may be comprised ofrespectively first parabolic reflecting surfaces 233, 242, and secondparabolic reflecting surfaces 234, 244. The elliptical sections 250 and260 may be comprised of, e.g., elliptical reflecting surfaces 252 and262. In FIG. 6B there is shown an alternative embodiment with anadditional two parabolic shell sections 232 and 236, with section 232comprising, e.g., a first parabolic reflecting surface 231 and a secondparabolic reflecting surface 234, and section 236 comprising, e.g., afirst parabolic reflecting surface 237, a second parabolic reflectingsurface 238 and a third parabolic reflecting surface 239.

Each of the reflecting shells 230, 240, 250 and 260 are arranged toreflect between them 100 percent of the light emitted from the ignitionpoint 21 within a section of a sphere from 11° to 55° from an axis ofrotation 310 generally aligned with the focus of the collector 225reflecting shells, with the shells 230, 240, 250 and 260 being generallysymmetric about this axis of rotation 310 also. By way of example, theembodiment of FIG. 6A shows an embodiment where essentially all of thelight in the portion of the sphere just described enters at least one ofthe shells 230, 240, 250 and 260. In the case of parabolic shellsections 230 and 240 is incident on the first reflecting surfaces 233,242, and either reflected towards the intermediate focus 40, or is thealso reflected off of the respective second reflective surface 234, 244to the intermediate focus. In the case of the elliptical shell sections250, 260 all of the light entering each such shell 250, 260 is reflectedto the intermediate focus, e.g., because the ellipses formed by thereflecting surfaces 252, 262 each have a first focus at the ignitionpoint 28 and a second focus at the intermediate focus 40.

Depending on the material of the respective reflecting surfaces 233,234, 242, 244, 252 and 262, the angle of incidence of the particularrays, the number of reflections in a given shell section 230, 240, 250and 260, a certain average efficiency of reflection will occur and alsodepending on the construction of the shells a certain percentage of theavailable light will enter each section 230, 240, 250 and 260, suchthat, as illustrated in FIG. 6A 19% is reflected and focused in shellsection 230 at an average total efficiency of 65%, 17% is reflected andfocused in shell section 240 at an average total efficiency of 75%, 43%is reflected in shell section 250 at an average total efficiency of 80%and 21% is reflected and focused in shell section 260 with an averagetotal efficiency of 91%.

FIG. 6B shows an alternative embodiment adding two more parabolic shellsections 232, 236. These added sections may serve, e.g., to collect morelight up to about 85% from the axis of rotation and at least one of theadded sections may have a first reflective surface 237, a secondreflective surface 238 and a third reflective surface 239. As can beseen from FIG. 6B, e.g., a ray 290 of emitted light from the source orignition point may just enter the parabolic reflective shell section 236and be reflected as ray 292 to the second reflective surface 238 andthen reflected as ray 294 to the third reflective surface 239 and thenform focused ray 296. Similarly a ray 300 may just enter the parabolicreflector shell 236 at the other extremity of the shall opening and alsobe reflected off of the first reflective surface 237 as ray 320 and thesecond reflective surface 237 as ray 304 and the very end of the thirdreflective surface as focused ray 306. In the case of, e.g., one of theparabolic shell sections, e.g., section 240 a ray 280 may just enterthis section 240 and be reflected off of the first parabolic reflectingsurface 242 as ray 282 and the very end of the second parabolicreflecting surface as focused ray 283, and another ray 284 may justenter the section 240 to be reflected off of the second reflectivesurface 244 as focused ray 286. In the case of one of the ellipticalshell sections, e.g., 250, a ray 308 emitted from the ignition point mayjust enter the shell section 250 and be reflected off of the ellipticalreflecting surface 252 as a focused ray 309, and a ray 318 just enterthe shell section 250 at the opposite side as ray 308 and be reflectedas focused ray 319.

Turning now to FIGS. 7 and 8 there is shown a plot of grazing angle ofincidence reflectivity for (1) a single layer Ruthenium reflectingsurface and (2) a Mo/Si bilayer stack with a Mo/Si 14 nm thick single Molayer and a 4 nm single Si layer and (3) a ten period multi-layer Mo/Sistack, with a pitch of 9.4 nm and a MO/Si thickness ratio of 22.5:1,having, e.g., 40 multi-layer stacks, each for grazing angles ofincidence of 5° and 15°. In each of the reflectors with Mo/Si amolybdenum substrate is assumed. In the case where spectral purity is apart of the specification for the delivered light, collectors can betuned to a certain wavelength, with some given bandwidth spread, e.g.,by using the reflective properties of, e.g., a nested shell collector tofavor reflectivity near a selected center wavelength, e.g., in theembodiments of FIGS. 6A and B.

FIG. 9 shows aspects of an embodiment of the disclosed subject matter.In this embodiment a collector assembly 330 may comprise, e.g., aportion of a spherical mirror reflecting surface 332, which may be anormal angle of incidence multi-layer stack, reflecting the lightproduced from the ignition point 28 to one of, e.g., three nestedelliptical shell sections 336, 338 and 340 in a nested elliptical shellcollector 334. Each of the shell sections 336, 338 and 340 may have areflecting surface 366, 368, 369 that is on the inside of a respectiveshell 360, 362, 364. As shown in FIG. 9, e.g., the shell section 336 mayreceive the light from a rim section 370 of the spherical mirror 332,the shell section 338 may receive the light from an intermediate sectionof the spherical mirror 332 and the shell section 340 receives the lightreflected from a central portion of the spherical mirror 332.

The shell sections 336,338 and 340 may be coated with a multi-layer ofMo/Si rather than the conventionally proposed thick single layer of Ru.According to aspects of an embodiment of the disclosed subject mattertwo reflections occur, e.g., one from the spherical mirror and one ineach shell, e.g., for shells having elliptical reflecting surfaces, atgrazing angles between about 5° and 15°, as can be seen from FIGS. 7 and8. This can, e.g., significantly reduce, e.g., a significant amount ofout of band EUV radiation, e.g., assuming that 13.5 is the desired band.Ru mirrors, e.g., in a Wolter-type configuration remain very reflectivefor both 13.5 nm and 11 nm at both 5° and 15° grazing angles ofincidence, whereas Mo/Si stacks of grazing angle of incidence reflectivecoatings, as shown in FIGS. 7 and 8 can be much more selective,especially around 15°.

The above described embodiment does not have the spatial purity of,e.g., a grating spectral purity filter, as has been proposed in the art,but it does have a significant advantage in reflectivity andpreservation of in-band EUV radiation over the other solutions, e.g., agrating filter, proposed in the art.

A lithium LPP EUV light source according to aspects of embodiments ofthe disclosed subject matter, could employ a solid stream of liquidlithium or a lithium droplet source. For a droplet source, the number ofatoms per droplet can be calculated and for a solid stream one canassume that only material within the focused beam constitutes a dropletat ignition, although, from a debris standpoint adjacent material in thestream may also form debris, particularly if struck by lower energylaser radiation in the skirts of the energy distribution of the focusedlaser beam.

Since it is contemplated that it is desirable for the droplet source tohave a droplet size matched to the focused beam, both types of targetsource can be considered to have the same droplet size given by adroplet diameter, d_(droplet). The volume of the droplet is then givenby: $\begin{matrix}{V_{droplet} = {\frac{1}{6}\pi\quad{d_{droplet}^{3}.}}} & \left\{ 1 \right\}\end{matrix}$Calculating the number of atoms per droplet follows from the density of,e.g., lithium and its atomic weight. The mass of the droplet is:M _(droplet) =V _(droplet)ρ_(lithium)  {2};where ρ_(lithium)=0.535 g/cm³ is the density of lithium, such that:M _(droplet)=0.280·d _(droplet) ³  {3};where the droplet diameter is in centimeters and the resulting mass isin grams. The number of atoms in the droplet is then given by dividingthe droplet mass by the atomic mass of lithium and converting unitsproperly: $\begin{matrix}{{N_{atoms} = {\frac{M_{droplet}(g)}{M_{{lithium}\quad{atom}}({amu})} \cdot \frac{1\quad{amu}}{1.6605 \times 10^{- 24}\quad g}}};} & \left\{ 4 \right\}\end{matrix}$where M_(lithium atom)=6.941 amu, i.e.,N _(atoms)=2.43×10²² ·d _(droplet) ³  {5};where the diameter of the droplet is in centimeters. Converting thedroplet diameter from centimeters to micrometers gives:N _(atoms)=2.43×10¹⁰ ·d _(droplet) ³  {6}.

The number of atoms per droplet versus droplet size is shown in FIG. 10Also shown in FIG. 10 is the number of 13.5 nm photons contained in,e.g., a single 40 mj pulse. The 40 mj pulse example assumes a 10%conversion efficiency into 4π steradians and a 400 mj laser pulse. Thenumber of 13.5 nm photons per pulse is given by: $\begin{matrix}{{N_{Photons} = \frac{13.5\quad{nm}\quad{{OpticalPulseEnergy}({mJ})}}{{{E_{Photon}({eV})} \cdot 1.6} \times 10^{- 16}\left( {{mJ}\text{/}{eV}} \right)}};} & \left\{ 7 \right\}\end{matrix}$where the 13.5 nm photon energy is 91.6 eV. The resulting number ofphotons for a 40 mj pulse is 2.72×10¹⁵. A, e.g., 50 μm droplet has onelithium atom for every 13.5 nm photon. Normally one could assumemultiple photons emitted from each emission element. This assumptionwould allow use of a smaller droplet diameter than 50 μm. A smallerdroplet diameter can be important because the lithium usage and lithiumdeposition rates, e.g., on the collector optics, scale as the cube ofthe droplet diameter.

Assuming that there is no lithium recovery, according to a possibleaspect of an embodiment of the disclosed subject matter, thencalculating, e.g., the yearly usage of lithium is given by the number ofpulses per year times the amount per pulse. Assuming, by way of examplea repetition rate, RR, and a duty cycle, DC, the resulting mass usageis, e.g.,:Mass Per Year=M_(droplet) ·RR·60 sec/min·60 min/hr·24 hr/day·365day/yr·DC  {8}i.e.,Mass Per Year=8.83×10⁻⁶ ·d _(droplet) ³ ·RR·DC  {9};where the droplet diameter is in micrometers and the resulting mass isin grams. For example, a system with no lithium recovery running at 6kHz with a droplet diameter of 50 μm running at 100% duty cycle for afull year would consume 6,622 grams or about a 12.3 liter volume oflithium. A droplet diameter of 25 μm under similar conditions wouldconsume only 828 grams or about 1.5 liters of lithium.

Assuming that the lithium droplet, once heated by the laser pulse,expands in all directions uniformly, the atomic flux will fall off asthe square of the distance from the laser-droplet interaction point(ignition site). The number of atoms emitted from the interaction pointper second is the number of atoms per droplet times the repetition rate:Total Atomic Emission=2.43×10¹⁰ ·d _(droplet) ³ ·RR  {10};where the droplet diameter is in micrometers and RR is the laserrepetition rate in Hz.

The atomic flux (atoms/cm²) through the surface an imaginary spherecentered at the ignition site will be the total atomic emission dividedby the surface area in centimeters: $\begin{matrix}{{{{Atomic}\quad{Flux}} = {1.93 \times 10^{9}\frac{d_{droplet}^{3} \cdot {RR}}{r_{sphere}^{2}}}};} & \left\{ 11 \right\}\end{matrix}$The resulting flux is in units of atoms/cm² s. FIG. 11 shows the rate oflithium influx onto the mirror surface vs. mirror radius for severaldroplet diameters, i.e., (1) 25 μm. (2) 55 μm, (3) 100 μm and (4) 200μm, assuming a 6 kHz repetition rate and 100% duty cycle.

In order to maintain high mirror reflectivity, the influx of lithiumonto the mirror surface can, e.g., be exceeded by the sputter rate oflithium, e.g., caused by incident helium ions. In addition, for longmirror lifetime the sputter rate of molybdenum by these same, e.g.,helium ions must then be many orders of magnitude slower than that for,e.g., lithium.

The required ratio of sputter rate of the first and second metals, e.g.,molybdenum to lithium, in order to achieve, e.g., a 1 year lifetime forthe multi-layer coated collector mirror can be calculated, e.g., byassuming use of, e.g., a multi-layer stack with 300 layer pairs, e.g.,so that erosion of, e.g., the first 200 layer pairs leaves a stillcomfortably effective 100 good pairs, i.e., still maintaining highreflectivity. Also assumed is a sputter rate for the silicon layers thatis much higher than that for the first metal, e.g., molybdenum layersand thus provides a negligible contribution to the mirror lifetime.

A typical EUV mirror can consist, e.g., of a layer pair of molybdenumand silicon with the molybdenum layer 2.76 nm thick, such that 200 pairsfor sacrificial erosion gives, e.g., 552 nm of molybdenum erosion beforeend-of-life for this mirror. For a 1-year useful life, the molybdenumsputter rate must be below 552 nm/year, i.e., 1.75×10⁻⁵ nm/sec.

The lithium sputter rate in terms of atoms per cm² per second (equal tothe lithium influx rate derived above) converts to nm/sec from thethickness of a monolayer of lithium, given the atomic number density oflithium per its mass density and atomic weight, with appropriate unitconversions, as follows: $\begin{matrix}{{{AtomicNumberDensity} = \frac{\rho_{lithium}\left( {g\text{/}{cm}^{3}} \right)}{{M_{lithiumatom}({amu})} \cdot \frac{1.6605 \times 10^{- 24}\quad g}{1{\quad\quad}{amu}}}};} & \left\{ 12 \right\}\end{matrix}$where ρ_(lithium)=0.535 g/cm³ and M_(lithium atom)=6.941 amu. Theresulting atomic number density for lithium is 4.64×10²² atoms/cm³. Ifthis number of lithium atoms where arranged in a cube with dimensions 1cm on each side, then the number of atoms along an edge per cm would bethe cube root of the atomic number density, 3.58×10⁷ atoms/cm. Theresulting monolayer thickness is 2.78×10⁻⁸ cm or 0.278 nm. The number ofatoms per cm² in a monolayer then is the square of the number of atomsalong an edge per cm: 1.28×10¹⁵ atoms/cm².

The number atoms of, e.g., lithium, removed by sputtering per secondmust match the influx rate given in Equation 11. Thus, the number ofmonolayers removed per second is equal to the influx rate divided by thenumber of atoms per cm² in a monolayer. Thickness removal rate is themonolayer removal rate times the thickness of a monolayer, i.e.,$\begin{matrix}{{ThicknessRemovalRate} = {{{MonolayerThickness}({nm})} \cdot {\frac{{InfluxRate}\left( {{atoms}\text{/}{cm}^{2}s} \right)}{{NumberofAtomsinaMonolayer}\quad\left( {{atoms}\text{/}{cm}^{2}} \right)}.}}} & \left\{ 13 \right\}\end{matrix}$Using the values for lithium:LithiumThicknessRemovalRate=2.17×10⁻¹⁶·LithiumInfluxRate(atoms/cm²s)  {14}with the resulting units of nm/sec. The lithium influx rate shown inFIG. 11 converts to a required lithium thickness sputter rate, shown inFIG. 12, for the same 1-4 droplet sizes, repetition rate and duty cycle.This result further highlights the need for a small droplet size and alarge mirror radius. Otherwise, the required sputter rate can becomeimpractical.

The required thickness sputter rate for lithium, can be compared to themaximum allowed thickness sputter rate for molybdenum, e.g., for a 1year collector lifetime. The data in FIG. 12 divided into the maximumallowed molybdenum sputter rate, 1.75×10⁻⁵ nm/sec is shown in FIG. 13for the same 1-4 droplet sizes, repetition rate and duty cycle.

The question is what is needed to create a molybdenum sputter rate 4 ormore orders of magnitude less than the lithium sputter rate. The sputteryield for lithium and molybdenum when attacked by helium ions isdiscussed, e.g., in W. Eckstein, “Calculated Sputtering, Reflection andRange Values”, [citation to publication?] ______, Jun. 24, 2002. Thissputter yield data versus ion energy is shown in FIG. 14 along with datafor silicon for ion energies of (3) lithium into Mo at Eth=52.7 eV, (2)helium into Si at Eth=10.1 eV and (1) helium into Li. As one can see, aproperly chosen helium ion energy will result in acceptable lithiumsputter yield and essentially no molybdenum sputter yield. A problem canarise, however, from the fact that one cannot control the incident ionenergy perfectly. That is, the energy spectrum of incident helium ionsis not a delta function. It is the spread of ion energies that must beassessed when determining the deferential sputtering between lithium andmolybdenum.

There are examples in the literature of RF Induction (RFI) plasmas whichcreate, e.g., an ion energy distribution that is Gaussian shaped with,e.g., a FWHM of 2.5 eV as discussed, e.g., in J. Hopwood, “IonBombardment Energy Distributions in a Radio Frequency Induction Plasma,”Applied Physics Letters, Vol 62, No. 9 (Mar. 1, 1993), pp 940-942.

The peak of the ion energy distribution can, e.g., be adjusted withproper choice of, e.g., electric field strength and helium pressure. Bychoosing, e.g., a peak ion energy of 20 eV, the helium ions have highsputter yield for lithium, but have energies safely below that of themolybdenum sputter threshold. In FIG. 15 there is shown a plot ofnormalized ion energy distribution (1 on the log scale and 2 on thelinear scale) centered on 20 eV and FWHM of 2.5 eV along with thesputter yields for (3) lithium, (4) silicon, and (5) molybdenum. One cansee that there are very few helium ions with energy above the molybdenumsputter threshold. To determine the sputter rate of molybdenum underthese conditions requires calculating the influx of helium ions neededto maintain the mirror surface clean of lithium atoms. A constantsputter yield of 0.2 atoms per ion can be assumed, since the bulk of thedistribution of helium ion energies falls within the region of nearlyconstant lithium sputter yield. $\begin{matrix}{{{HeliumIonInflux}\quad\left( {{ions}\text{/}{cm}^{2}s} \right)} = {\frac{{LithiumInflux}\quad\left( {{atoms}\text{/}{cm}^{2}s} \right)}{{SputterYeild}\left( {{atoms}\text{/}{ion}} \right)}.}} & \left\{ 15 \right\}\end{matrix}$Thus, the helium ion density must be 5 times the value of lithium influxdensity shown for various conditions in FIG. 11.

This helium ion influx expressed in Equation 15 may be considered to bethe bare minimum, assuming, e.g., the lithium does not deposit perfectlyuniformly. In this event a higher total sputter rate may be required,e.g., to ensure that islands of lithium do not develop. On the otherhand, other researchers have shown that the ejection of material from anLPP plasma tends to travel toward the laser source. One can, therefore,e.g., arrange the system such that the laser illuminates the lithiumdroplet from a direction away from the collector, or through an aperturein the collector mirror that causes much of this debris to not strikethe collector mirror. Thus, the total lithium load on the mirror may bereduced from the total theoretical amount striking the mirror.

Knowing the total flux of helium ions and assuming a Gaussian energydistribution with a peak at 20 eV and a FWHM of 2.5 eV, the integral ofa normalized Gaussian distribution is √{square root over (2πσ²)} whereσ² gives a variance of the distribution related to the FWHM by:$\begin{matrix}{\sigma^{2} = {\frac{({FWHM})^{2}}{4{\ln(4)}}.}} & \left\{ 16 \right\}\end{matrix}$The integral of a normalized Gaussian then is$\sqrt{\frac{{\pi({FWHM})}^{2}}{2{\ln(4)}}},$so that the peak current density of helium ions is given by:$\begin{matrix}{{{PeakHeliumCurrentDensty}\left( {{ions}\text{/}{cm}^{2}s\quad{per}\quad{eV}} \right)} = {\frac{{HeliumIonInflux}\quad\left( {{ions}\text{/}{cm}^{2}s} \right)}{\sqrt{\frac{{\pi({FWHM})}^{2}}{2{\ln(4)}}}}.}} & \left\{ 17 \right\}\end{matrix}$Taking the case of a 25 μm droplet with a mirror radius of 10 cm, thepeak helium current density must be 3.38×10¹⁵ ions/cm² s per eV in orderto sputter a total of 1.88×10¹⁵ lithium atoms/cm² s. This helium currentdensity distribution (1) is plotted in FIG. 16 on a log scale, with (2)silicon sputter density and (3) lithium sputter density, along withempirically determined sputter yield of (4) lithium, (5) silicon, and(6) molybdenum and the product of these functions times the ion currentdensity. A surprisingly beneficial result of this analysis shows thatthe peak sputter density for molybdenum is 3.5×10⁻²⁰⁵ atoms/cm² s per eV(not shown on graph), an incredibly small value. In fact, even the peaksilicon sputter density is more than 3 orders of magnitude smaller thanthat for lithium.

The integral of these sputter densities over all helium ion energiesgives the total sputter rate. These integrals are shown respectively asdashed curves (1) for lithium and (2) for silicon, in FIG. 17. Theintegrated lithium sputter density is 1.88×10¹⁵ atoms/cm² s, matchingthe lithium influx rate. The integrated silicon sputter density is9.17×10¹⁰ atoms/cm² s. The integrated molybdenum sputter density is1.16×10⁻²⁰⁵ atoms/cm² s. Therefore, differential sputter rates betweenmolybdenum and lithium are so low that, e.g., less layers of thecollector mirror need be employed, e.g., many less that a previouslyanticipated 300 base pair mirror concept. A single molybdenum layer willlast more than a year under these conditions and the assumptions of thissputter yield model. This performance could be even more improved usinga debris shield between the ignition spot and the collector main mirroror main an secondary mirrors, but the debris shield, as seen from theseresults, may also be totally eliminated, at least for a lithium target.This type of stimulated plasma induced ionized sputtering of debris fromthe EUV optics, especially for a lithium target, as seen from the above,could even allow for use of other target types, e.g., a moving tape orother type of moving solid target system. Helium ion sputtering can bearranged such that it removes the lithium atoms from the collectormirror at a sufficient rate while sputtering molybdenum at a low enoughrate for far greater than 1 year lifetime.

Sputtering of molybdenum by, e.g., lithium ions must also be consideredin the embodiment of the disclosed subject matter being discussed,since, e.g., there will be lithium ions formed a debris from theignition plasma which do not reach the optic surface, but which will beavailable to the sputtering plasma and will be accelerated toward themirror surface with a similar energy distribution as the helium ions.The literature also provides data on sputter yield of lithium andmolybdenum with lithium ions. This data is shown in FIG. 18 in curve 1for lithium at Eth=36.3 eV, along with the same normalized lithium ionenergy distribution as was used for the helium ions. To calculate themolybdenum sputter density from lithium the total lithium ion influxmust be known. Unlike this calculation for helium (Equation 15) it isnot clear what the total lithium influx will be, however, a conservativechoice would the total lithium atomic influx generated by the LPPignition plasma. Using Equation 17 and the assumptions of a 25 μmdroplet and a 10 cm mirror radius, 1.88×10¹⁵ lithium ions/cm² s would beincident on the mirror, and the peak lithium ion current density is7.06×10¹⁵ lithium ions/cm² s per eV, with the assumption of a 2.5 eVFWHM spread in incident ion energy, which, when multiplied by thesputter yield for molybdenum and integrated over all ion energies, givesa total molybdenum sputter density of 2.54×10⁴⁸ atoms/cm² s. This ismuch higher than that for helium ions, but still much, much lower thanthe rate required for one year of useful life.

The molybdenum sputter density with lithium ions can be converted tothickness loss rate by using Equations 12 and 13. For molybdenum:

ρ_(moly)=10.2 g/cm³

M_(moly atom)=95.94 amu=1.59×10⁻²² g

Moly Atomic Number Density=6.40×10²² atoms/cm³

Moly Monolayer Thickness=2.50×10⁻⁸ cm=0.250 nm

Moly Monolayer Atomic Density=1.59×10¹⁵ atoms/cm²

Thus, the sputter thickness rate for molybdenum, when attacked bylithium atoms, is 3.99×10⁻⁶⁴ nm/sec or 1.25×10⁻⁵⁶ nm/year. This alsoleads to the conclusion that the above noted beneficial results of thesputtering plasma ionized cleaning of the EUV optics by, e.g., heliumion sputtering are still realizable even with, e.g., lithium sputteringof molybdenum.

An additional beneficial result is the reconsideration of the previouslyproposed use of, e.g., a ruthenium capping layer on, e.g., themulti-layer mirror. A ruthenium capping layer has been proposed toprevent EUV-assisted oxidation of the first silicon layer in the Mo/Sistack. Multi-layer mirrors are usually terminated with a silicon layerrather than a molybdenum layer because the molybdenum layer wouldquickly oxidize once exposed to room air. Applicants, before the aboveanalysis regarding sputtering plasma cleaning of the EUV optics hadconsidered, e.g., a multi-layer mirror terminated with silicon, with theexpectation that the first layer of silicon would be eroded to exposethe first layer of molybdenum or a ruthenium capping layer to avoidoxidation of a first layer of molybdenum if that approach was taken. Thesuper-slow erosion rate of molybdenum, and a similar expected lowerosion rate for ruthenium allows for use of a ruthenium capping layerexpected to last for the useful life of the mirror. This results in noloss of the first layer of silicon, and no need to worry about whathavoc the sputtered silicon atoms might cause, and no oxidation problemswith an exposed molybdenum layer. The sputter yield of ruthenium withlithium and helium, although expected to be similar to that ofmolybdenum, since ruthenium has a higher atomic mass than molybdenum,remains to be determined.

The minimum RF power needed to create the desired sputtering plasma ator near the optic surface can be calculated by assuming, e.g., thatevery helium ion that is created strikes the collector mirror, whichwill underestimate the required RF power, but should give an order ofmagnitude estimate. Each helium ion that strikes the collector mirrorrequires 24.5 eV to ionize, and according to the above example of anembodiment of the disclosed subject matter has to have an averagekinetic energy of 20 eV when it reaches the collector mirror. These twoenergy values times the required influx of helium ions, 9.40×10¹⁵ions/cm² s from Equation 15, gives the plasma power. Converting energyunits from eV to J gives a minimum plasma power density of 66.9 mW/cm².Multiplying by the half the surface area of the 10 cm radius mirror, 628cm², gives 42 W of minimum total plasma power. Assuming conservativelythat only 1% of the plasma power is effectively used, the requiredplasma power calculated is 4.2 kW, which is acceptable, especiallyconsidering the very large area over which this power can be dissipated.This estimate of plasma power compares to the previous assumptions of400 mJ per pulse at 6 kHz LPP laser power, 2.4 kW of laser power andassuming the collector mirror subtends π steradians, it will be exposedto half of this laser power, i.e., 1.2 kW. The thermal load from the LPPis similar to the thermal load of the plasma cleaning. The sum of thetwo powers is 5.4 kW, resulting in a power density on the mirror of 8.6W/cm². Applicants believe that a collector mirror exposed to a 10 W/cm²or less power density is easily cooled, e.g., with water channels alongthe back of the mirror, or between the grounded shield and the mirror.

If the plasma power effectivity is more like 10%, then the total powerdensity onto the mirror is only 2.6 W/cm², making it possible toradiatively cool the mirror, according to Stefan's law of radiation,which states that the power radiated per square meter from a black bodyat temperature T is given by:P=5.67×10⁻¹² ·T ⁴  {18};where temperature is in Kelvin and the resulting power density is inW/cm², which is plotted in FIG. 19. A temperature in excess of 500° C.would be required to radiate all of this incident power, so activecooling of the collection mirror appears to be required in order toprevent damage to the multi-layer stack.

Turning now to FIG. 20 there is shown an schematically an apparatus andmethod according to an embodiment of the disclosed subject matter forreclaiming damaged EUV optics, e.g., those that have lost reflectivity,e.g., due to deposition of material on the reflective surface, e.g.,carbon and/or carbon based molecules, which may come from, e.g.,contamination entering the EUV plasma chamber or from sputtering orphotonic removal from layers of the multi-layer reflective stack coatedon reflecting surfaces un the EUV apparatus. As can be seen in FIG. 20 aphoto-chemical cleaning apparatus 400 may include a chamber, withinwhich may be mounted, e.g., a collector holding jig 402 that is adaptedto hold a collector for cleaning. Also included may be, e.g., a sourceof photonic energy, e.g., a DUV light source 410, with the collectorholding jig 402 and the light source 410 arranged so that the light fromthe light source 410 simulates light coming from a point source at thefocus of the collector, e.g., the ignition site 28 discussed above, suchthat the collector 404 is irradiated as if by light from a targetignition site.

According to an embodiment of the disclosed subject matter, e.g., thechamber 401 may first be purged by the use of nitrogen provided to thechamber through N₂ valve and then evacuated from the chamber 401 usinggas exit valve, followed by the introduction of a fluorine containinggas, e.g., molecular F₂ or NF₃. The collector 404 may then be subjectedto irradiation by the light source, e.g., DUV light at a range of λbetween, e.g., 160-300 nm, e.g., from a KrF excimer laser at 193 nm,e.g., in a MOPA configuration for high power at about 40 W, with a pulserepetition rate at about 4 kHz. This can serve, e.g., to stimulate theproduction of, e.g., fluorine based carbon materials, e.g., CF₄, e.g.,in a gas phase, which can then be evacuated from the chamber 401 throughthe gas exit valve 420 under a second nitrogen purge.

An alternative of a KrF DUV light source could be, e.g., a commerciallyavailable DUV lamp, e.g., a KrCl DUV lamp.

Applicants expect that thicknesses of about 3.5 nm carbon atomdeposition on an EUV optic, e.g., a collector reflective surface canreduce reflectivity by about 5% and a 10 nm deposition by about 14%.Such levels of thickness of deposit are expected to be removed from,e.g., the collector optics reflective surfaces under treatment influorine with selected concentrations and the above referenced level ofDUV light for a selected time. The process could also employreplenishing the fluorine supply with a gas flow control valve (notshown) to maintain, e.g., a desired concentration of fluorine during thecleaning process.

Applicants herein also propose according to an aspect of an embodimentof the disclosed subject matter that other types of barrier materialsmay be used in the multi-layer reflecting mirror stacks to help improvethe thermal stability and reflectivity of, e.g., Mo/Si reflectivestacks, e.g., optimized for 13.5 nm EUV light reflectivity. To promotesmoothness of very thin, e.g., 1 nm barrier layers, that are compatiblewith, e.g., Mo/Si and perhaps also MoSi₂, retaining the appropriatelevels of transparency to, e.g., 13.5 nm light, applicants propose theuse of inter-diffusion barrier layers comprising carbides selected fromthe group comprising ZrC, NbC, SiC, borides, e.g., selected from thegroup ZrB₂, NbB₂, disilicides selected from the group comprising ZrSi₂,NbSi₂ and nitrides BN, ZrN, NbN and Si₃N₄. Other such layers couldinclude yttrium, scandium, strontium compounds and/or these metals inpure form. Among the above, the carbides and borides mentioned arepreferred due to the ability to create smoother diffusion barrier layerswith such materials.

According to aspects of an embodiment of the disclosed subject matterapplicants contemplate multi-layer stacks, including e.g., MoSi₂/Si,Mo₂C/Si, Mo/C/Si/C and Mo/X/Si/X, where the first two are MLMs whereMoSi₂ or Mo₂C is used in place of the Mo normally used in normal Mo/Simirror coatings, with no inter-diffusion barriers. The other two arewith the so-called inter-diffusion barriers, where C refers to carbonand X refers to a suitable material, including further compounds, e.g.,the above noted borides, disilicides, and nitrides as the X materials.Nitrides are currently preferred embodiments according to applicants forinter-diffusion barrier layers in the applications according toembodiments of the disclosed subject matter. Mo₂Si/Si is described inthe paper Y. Ishii et al. “Heat resistance of Mo/Si, MoSi₂/Si, andMo₅Si₃/Si multilayer soft x-ray mirrors”, J. Appl. Phys. 78, (1995) p.5227.

Helium has high transparency to EUV, which makes it a good choice for abuffer gas for which a transmission of 90% is representative. Based onthe partial pressures required for efficient sputtering, a few mTorr,helium buffer gas transmission would be nearly 100%. A possiblecollector multi-layer surface could comprise, e.g., 300 coating pairsinstead of the normal 90 pairs. The extra pairs would not improve thereflectivity over a 90 pair mirror, but instead these extra layers can,if required, get used once the top layers are eroded away. With a 300pair mirror the sputter rate differential between lithium and the mirrorneed not be so high that a single mirror layer lasts for, e.g., monthsat a time. Instead three could be, e.g., an extra 210 layer pairs worthof mirror erosion that can be sustained.

Lithium chemical compounds that might be generated in the LPP vessel,e.g., LiH, LiOH, Li₂CO₃, etc., can have melting points in excess of 600°C. and thus may not be evaporated from the mirror. These could even formin certain cases, e.g., a crust over the lithium which deposits on themirror surfaces. These could, however, very effectively be sputtered bythe sputtering ion plasma, e.g., containing the ionized He atoms, or maybe sputtered by lithium itself in the form of high speed lithium ionsand atoms ejected from the plasma that impinge on the reflectingsurface.

The sputter rate required to stay ahead of the lithium deposition couldbe much higher in an EUV light source than the literature indicates istypically practiced, e.g., in modern deposition and etch machines, whichis at lease part of a reason for, e.g., a combined approach to keepingthe, e.g., lithium off of the mirror surfaces. According to an aspect ofan embodiment of the disclosed subject matter applicants contemplateusing evaporation to remove the bulk of the lithium while employing avery light sputter rate to remove the inevitable lithium and carboncompounds deposited on the mirror surface. However, even a very lightsputtering plasma impinging on at least the main and secondaryreflecting surfaces could have the same beneficial carbon and otherlithium compound removal properties. Employing this idea beyond theintermediate focus, e.g., in the illuminator reflective surfaces andalso the projection reflecting surface may also prove beneficial toremove debris that happens to reach the lithography tool reflectingsurfaces. In the lithography tool itself, due to, e.g., smallerdeposition rates the thermal load and sputtering rate may besufficiently low for this to be effective.

Sputtered lithium and lithium compounds along with lithium ejected fromthe plasma that does not collect on the reflecting surface may betrapped in cold fingers [not shown] contained in the EUV light sourcevessel, e.g., in the form of cooled, e.g., water cooled fins or platesextending from the inside walls of the vessel, and out of the opticalpath from the collector to the intermediate focus.

In the case of, e.g., tin as the source element it may be possible touse, e.g., a hydrate of the metal, e.g., SnH₄, which is a vapor at roomtemperature, along with a hydrogen plasma for cleaning the collector ina tin-based LPP source. Hydrogen has high 13.5 nm transmission and theresulting SnH₄ could be pumped away rather than trapped on cold fingerslike the lithium.

Applicants have examined, e.g., the stopping power of helium and argonagainst both tin and lithium ions. The results are shown in FIGS. 21Aand B. The two graphs have the same data, just different scales. Lines500, 502 and 503, for tin at different measured distances from a sourceplasma, respectively 96.5 cm, 61 cm and 32.5 cm with solid being heliumbuffer and dashed being argon buffer. The lines 506 are for lithium. Ifpressure*distance product scaling were applied, these three sets of fortin data would fall approximately on top of each other.

Applicants have also determined that Argon has at least 10 times higherstopping power than helium for a given gas pressure. Also, lithium canbe stopped with less buffer gas than tin. And, scaled to the trueworking distance of an LPP collector (˜10 cm), the required bufferpressure, even with argon, will need to be in the range of about 10 mTfor tin. Since xenon and tin have nearly the same atomic mass,applicants expect that the required buffer pressure for a xenon LPPwould also be in the range of 10 mT. Such a high buffer gas pressure canpresent EUV self-absorption problems for xenon and tin. But not forlithium, both because of the lower buffer pressure requirement and alsothe lower EUV absorption of lithium.

In continuing to examine the stopping power of a buffer against, e.g.,the fast ions produced by the LPP, using, e.g., a Faraday cup to collectand measure the ions at a known distance through a known aperture sizeat different increasing buffer gas pressure this Faraday cup signaldecreased, giving a measure of the ion stopping power. The results fortin and lithium are shown below in FIGS. 22A-E. FIGS. 22A and 22B showthe raw Faraday cup signal vs. time for, respectively tin and lithium.In FIGS. 22C and D these signals are plotted vs. ion energy usingtime-of-flight respectively for tin and lithium. In FIG. 22E the areaunder these curves is plotted vs. a pressure*distance product of thebuffer gas, with the lower plot line (1) being tin and the upper (2)being lithium.

A surprising result of this analysis by applicants was that the lastgraph shows the Faraday cup signal vs. buffer gas P*D product for bothtin and lithium being about the same for both elements. Applicantsbelieve this is explainable in that the analysis was not reallymeasuring the loss of ions captured by the Faraday cup, but instead wasmeasuring the neutralization of the ions by the buffer gas, so-calledelectron capture by the ions. If an ion is neutralized, it will notregister in the Faraday cup. This can be explained, e.g., because tinmight have a larger electron capture cross-section than lithium,especially considering that the tin ion is highly charged, 7-11 timesionized and the lithium can, at most, be 3 times ionized. The stoppingpower result shown in FIG. 22E can be considered an overestimate of thebuffer gas stopping power in that it can be no better than the valuepredicted by these curves.

Taking the observed values of the stopping power as the upper limit onecan calculate the necessary pressure of argon buffer gas to extend thecollector mirror lifetime to 100 B pulses. Starting with the result fromEngineering Test Stand (ETS) built by the EUV LLC, which reported thatone multi-layer mirror pair is eroded for every 15 M pulses with a xenonLPP and a collector distance of 12 cm, and assuming that thereflectivity of a multi-layer mirror is not significantly degraded until10 layer pairs are removed, the ETS collector mirror had a lifetime of150M pulses compared to a requirement of 100 B pulses. This leads to theconclusion that a reduction of 666× in erosion rate is necessary. On theplot in FIG. 22E a P*D product of approximately 500 mT*cm would berequired to achieve this level of reduction. A working distance of,e.g., 12 cm gives, e.g., a need for an argon pressure of 42 mT. Thisalso can result in the conclusion that lithium is the better targetover, e.g., xenon, since for xenon LPP, a buffer pressure of 42 mT isnot very satisfactory due to the strong EUV absorption of the xenoncaught up in the argon buffer. For lithium, however, this amount ofbuffer pressure is no problem for lithium absorption. Tin may also besatisfactory, depending on, e.g., the vapor pressure and evolution rateof SnH4 from the surface of the collector mirror. A relatively largebuffer gas pressure seems, therefore, to be a requirement, which leadsto the conclusion that xenon is not a good target, tin may be, butlithium appears to be the best.

Applicants have also determined that even if the effectiveness ofheating the collector reflective surfaces is impacted by the fact thatthe material being evaporated needs to have, e.g., a certain thickness,e.g., 50 Å, e.g., about 10 monolayers, before published values of vaporpressure are realized, i.e., the material, e.g., lithium may be harderto evaporate directly off of the surface of the mirror, nevertheless,the transmittance of such a thickness of lithium on the mirror surfacesis about 95%, and about 90%, double-pass, so that such a layer on themirror would not significantly detract from the overall CE, e.g., at13.5 nm. In addition, such a layer of “evaporationless” lithium, mayactually be beneficial in that it may be able to protect the collectormirror from the onslaught of high-speed lithium atoms and ions. Thislithium layer will be sputtered instead of the molybdenum layer of themulti-layer mirror. Xenon, because it is a gas, will not form such aprotective layer and a tin layer, because of its very high EUVabsorption, would only be 52% transmitting.

Given that the sputter yield of lithium against molybdenum is much lessthan the sputter yield of xenon against molybdenum, e.g., for ionenergies around 1 keV (the sputter rates tend to saturate above thisenergy level): Incident Ion Target Material Lithium Xenon Lithium 0.21?? Molybdenum 0.081 1.45xenon will sputter molybdenum at 18 times higher rate than lithium. Thisdifference alone would give a 2.7 B pulse collector lifetime withoutchanging anything else. The “evaporationless” steady state thin layer oflithium may provide the remaining 37× reduction in sputter rate. Even ifit does not, the EUV LLC concept of producing a mirror with ˜100 extrasacrificial layer pairs could add a 10× increase in lifetime, e.g., to27 B pulses, which combined with the lower erosion from lithium couldgive a collector lifetime of 100 B pulses.

Applicants have also examined the effectiveness of electrostaticprotection of the collector mirror. The concept has been proposed in theliterature, i.e., to generate an electric field between the source LPPand the collector mirror such that the energetic ions must climb up apotential well as they travel toward the mirror. This potential well canbe made deep enough that the ions loose all of their kinetic energybefore reaching the mirror. In fact, they are turned around and sentpacking back down the potential well, never reaching the mirror.Applicants have discovered, however, that attempting to do this byrunning an electrical connection through the vessel to the collectormirror was ineffective due to the target bias dropping to near zero uponpulsing the laser, which was determined to be the result of a high peakcurrent required to maintain the bias voltage and the large lead wirerequired, thus dropping all of the voltage along the inductance of thewire. To correct this problem applicants then installed capacitorsinside the vacuum vessel and constructed low inductance buss workbetween ground and the target plate. Inductance was measured by placinga copper sheet up and around the target and attached to ground. Bycharging the capacitors to a low voltage and discharging them bypressing the copper sheet against the target applicants measured theringing voltage waveshape and inferred the inductance. The result was104 nH with a 697 ns half-period discharge waveshape. This dischargeperiod is much longer than the laser pulse and subsequent EUV emissioninitially causing concern whether the bias could be maintained duringthe critical period when the ions are created and leave the plasmaregion (˜20 ns). Applicants determined, however, that such short timescales were unimportant. What is important is, e.g., to maintain orreestablish the target bias, e.g., in a time scale that is, e.g., shortcompared with the travel time of the ions from the target to the mirror.With the present geometries the ion travel time is about 2.5 μs, so acircuit half-period of 0.7 μs should be sufficient.

In testing this arrangement applicants were surprised to find that thefull 0.47 pF capacitance was drained of its −1000V potential in a timescale almost exactly the same as during the inductance measurement usingthe copper strap. Applicants determined that the laser pulse initiates adischarge between the target plate and the vessel wall. This dischargecompletes the circuit between the capacitor's high voltage terminal andground, thus draining the capacitors as if a copper strap had beenplaced across them. Evidently, the events that unfold during, andimmediately after, the laser pulse, a plasma being created at the targetpoint and this plasma radiating a large amount of hard UV and EUVradiation throughout the vessel. The energy of most of these photons isabove the work function of the metals inside the vessel and thusphotoelectrons are created at all the metal surfaces. These photons arealso energetic enough to ionize any gas atoms that exist in the vessel.In this case argon was used as the buffer gas and it is easily ionizedby the hard UV and EUV radiation produced by the LPP. And finally,electrons and ions are created in the LPP and stream outward into thevolume of the vessel. Except for those ions that are attracted to thebiased target plate. They strike the plate and create secondaryelectrons. Essentially, the creation of a discharge between two metalplates held at a potential between each other occurs as if thearrangement were a laser-triggered discharge switch.

There still is some possibility of making an effective electrostaticrepulsion, but it becomes a bit more complicate and isn't reallyelectrostatic. The idea is to pulse the bias such that it is presentonly after the initial events of the laser pulse. In only a few 100's ofns most of the electrons will have collided with the vessel wall, and ofcourse the radiation will be gone. At this time it might be possible toapply a bias and repel, or attract, the ions away from the collectormirror.

It will be understood by those skilled in the art that an apparatus andmethod is disclosed which may comprise, a multi-layer reflecting coatingforming an EUV reflective surface which may comprise an inter-diffusionbarrier layer which may comprise a carbide selected from the group ZrCand NbC or a boride selected from the group ZrB₂ and NbB₂ or adisilicide selected from the group ZrSi₂ and NbSi₂ or a nitride selectedfrom the group BN, ZrN, NbN, BN, ScN and Si₃N₄. The apparatus and methodmay comprise an EUV light source collector which may comprise acollecting mirror which may comprise a normal angle of incidencemulti-layer reflecting coating; an inter-diffusion barrier layercomprising a material selected from the group comprising a carbideselected from the group ZrC and NbC, or a boride selected from the groupZrB₂ and NbB₂ or a disilicide selected from the group ZrSi₂ and NbSi₂ anitride selected from the group BN, ZrN, NbN, BN, ScN and Si₃N₄. Themulti-layer reflecting coating may comprise a capping layer which maycomprise ruthenium. The multilayer reflecting coating may comprisemultiple alternating layers of an absorber and a separator. The absorbermay comprise molybdenum and the separator may comprise silicon. Thecapping layer may comprise molybdenum. The sputter thickness rate forsputtering of the capping layer material by material comprising a plasmaproduction material may be at or below a rate that will result in thecapping layer material sustaining such sputtering for greater than aselected lifetime. The multi-layer reflecting coating may comprise acapping layer which may comprise a material other than the absorbermaterial and the separator material selected to have a sputter thicknessrate that will sustain sputtering by a material comprising a plasmaproduction material at or below a rate that will result in a singlelayer of the capping layer material sustaining such sputtering forgreater than a selected time and to have more favorable properties whenexposed to ambient or operating environments than those of the absorbermaterial or the separator material. The multilayer reflecting coatingmay comprise multiple alternating layers which may comprise amulti-layer mirror optimized for a selected nominal center wavelength inthe EUV range. The apparatus and method may comprise a multi-layerreflecting coating forming an EUV reflective surface which may comprisean inter-diffusion barrier layer comprising yttrium, scandium orstrontium. The apparatus and method may comprise an EUV light sourcecollector which may comprise a collecting mirror which may comprise anormal angle of incidence multi-layer reflecting coating; aninter-diffusion barrier layer comprising a material comprising yttrium,scandium or strontium. The multilayer reflecting coating may comprisemultiple alternating layers which may comprise MoSi₂/Si or Mo₂C/Si, withor without an inter-diffusion barrier layer or inter-diffusion barrierlayers. The multilayer reflecting coating may comprise multiplealternating layers which may comprise Mo/X/Si/X where X may comprise aninter-diffusion barrier layer which may comprise C or a carbide selectedfrom the group ZrC and NbC, or a boride selected from the group ZrB₂ andNbB₂, or a disilicide selected from the group ZrSi₂ and NbSi₂ or anitride selected from the group BN, ZrN, NbN, BN, ScN and Si₃N₄.

Those skilled in the art will appreciate that the above referencespreferred embodiments of the disclosed subject matter and aspectsthereof are not meant to be exclusive and other modifications andadditions to the above referenced embodiments may be made withoutdeparting from the spirit and scope of the inventions disclosed in thepresent application. The appended claims, therefore, should not beconsidered to be limited to the above disclosed embodiments and aspectsbut should include with the scope and spirit of the claims the recitedelements and equivalents of the recited elements. By way of example,other target material and multi-layer reflective coating metals may havesimilar relationships as discussed above to allow for the continuouscleaning by, e.g., sputtering, e.g., of ions, e.g., induced by thecreation of a sputtering plasma in the vicinity of the optic reflectingsurface(s), which ions may also be, e.g., other than helium, e.g., H, Nor O. Also, e.g., the heating mechanism for the reflecting surface couldbe a heat lamp directed at the reflective surface. Other such changesand additions may be appreciated by those skilled in the art.

While the particular aspects of embodiment(s) of the {TITLE] describedand illustrated in this patent application in the detail required tosatisfy 35 U.S.C. §112 is fully capable of attaining any above-describedpurposes for, problems to be solved by or any other reasons for orobjects of the aspects of an embodiment(s) above described, it is to beunderstood by those skilled in the art that it is the presentlydescribed aspects of the described embodiment(s) of the subject matterclaimed are merely exemplary, illustrative and representative of thesubject matter which is broadly contemplated by the claimed subjectmatter. The scope of the presently described and claimed aspects ofembodiments fully encompasses other embodiments which may now be or maybecome obvious to those skilled in the art based on the teachings of theSpecification. The scope of the present [TITLE] is solely and completelylimited by only the appended claims and nothing beyond the recitationsof the appended claims. Reference to an element in such claims in thesingular is not intended to mean nor shall it mean in interpreting suchclaim element “one and only one” unless explicitly so stated, but rather“one or more”. All structural and functional equivalents to any of theelements of the above-described aspects of an embodiment(s) that areknown or later come to be known to those of ordinary skill in the artare expressly incorporated herein by reference and are intended to beencompassed by the present claims. Any term used in the Specificationand/or in the claims and expressly given a meaning in the Specificationand/or claims in the present application shall have that meaning,regardless of any dictionary or other commonly used meaning for such aterm. It is not intended or necessary for a device or method discussedin the Specification as any aspect of an embodiment to address each andevery problem sought to be solved by the aspects of embodimentsdisclosed in this application, for it to be encompassed by the presentclaims. No element, component, or method step in the present disclosureis intended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element in the appended claims is to be construed under theprovisions of 35 U.S.C. §112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or, in the case of amethod claim, the element is recited as a “step” instead of an “act.”

It will be understood also be those skilled in the art that, infulfillment of the patent statutes of the United States, Applicant(s)has disclosed at least one enabling and working embodiment of eachinvention recited in any respective claim appended to the Specificationin the present application and perhaps in some cases only one. Forpurposes of cutting down on patent application length and drafting timeand making the present patent application more readable to theinventor(s) and others, Applicant(s) has used from time to time orthroughout the present application definitive verbs (e.g., “is”, “are”,“does”, “has”, “includes” or the like) and/or other definitive verbs(e.g., “produces,” “causes” “samples,” “reads,” “signals” or the like)and/or gerunds (e.g., “producing,” “using,” “taking,” “keeping,”“making,” “determining,” “measuring,” “calculating” or the like), indefining an aspect/feature/element of, an action of or functionality of,and/or describing any other definition of an aspect/feature/element ofan embodiment of the subject matter being disclosed. Wherever any suchdefinitive word or phrase or the like is used to describe anaspect/feature/element of any of the one or more embodiments disclosedherein, i.e., any feature, element, system, sub-system, component,sub-component, process or algorithm step, particular material, or thelike, it should be read, for purposes of interpreting the scope of thesubject matter of what applicant(s) has invented, and claimed, to bepreceded by one or more, or all, of the following limiting phrases, “byway of example,” “for example,” “as an example,” “illustratively only,”“by way of illustration only,” etc., and/or to include any one or more,or all, of the phrases “may be,” “can be”, “might be,” “could be” andthe like. All such features, elements, steps, materials and the likeshould be considered to be described only as a possible aspect of theone or more disclosed embodiments and not as the sole possibleimplementation of any one or more aspects/features/elements of anyembodiments and/or the sole possible embodiment of the subject matter ofwhat is claimed, even if, in fulfillment of the requirements of thepatent statutes, Applicant(s) has disclosed only a single enablingexample of any such aspect/feature/element of an embodiment or of anyembodiment of the subject matter of what is claimed. Unless expresslyand specifically so stated in the present application or the prosecutionof this application, that Applicant(s) believes that a particularaspect/feature/element of any disclosed embodiment or any particulardisclosed embodiment of the subject matter of what is claimed, amountsto the one an only way to implement the subject matter of what isclaimed or any aspect/feature/element recited in any such claim,Applicant(s) does not intend that any description of any disclosedaspect/feature/element of any disclosed embodiment of the subject matterof what is claimed in the present patent application or the entireembodiment shall be interpreted to be such one and only way to implementthe subject matter of what is claimed or any aspect/feature/elementthereof, and to thus limit any claim which is broad enough to cover anysuch disclosed implementation along with other possible implementationsof the subject matter of what is claimed, to such disclosedaspect/feature/element of such disclosed embodiment or such disclosedembodiment. Applicant(s) specifically, expressly and unequivocallyintends that any claim that has depending from it a dependent claim withany further detail of any aspect/feature/element, step, or the like ofthe subject matter of what is recited in the parent claim or claims fromwhich it directly or indirectly depends, shall be interpreted to meanthat the recitation in the parent claim(s) was broad enough to cover thefurther detail in the dependent claim along with other implementationsand that the further detail was not the only way to implement theaspect/feature/element claimed in any such parent claim(s), and thus belimited to the further detail of any such aspect/feature/element recitedin any such dependent claim to in any way limit the scope of the broaderaspect/feature/element of any such parent claim, including byincorporating the further detail of the dependent claim into the parentclaim.

1. An apparatus comprising: a multi-layer reflecting coating forming anEUV reflective surface comprising: an inter-diffusion barrier layercomprising a carbide selected from the group ZrC and NbC.
 2. Anapparatus comprising: a multi-layer reflecting coating forming an EUVreflective surface comprising: an inter-diffusion barrier layercomprising a boride selected from the group ZrB₂ and NbB₂.
 3. Anapparatus comprising: a multi-layer reflecting coating forming an EUVreflective surface comprising: an inter-diffusion barrier layercomprising a disilicide selected from the group ZrSi₂ and NbSi₂.
 4. Anapparatus comprising: a multi-layer reflecting coating forming an EUVreflective surface comprising: an inter-diffusion barrier layercomprising a nitride selected from the group BN, ZrN, NbN, BN, ScN andSi₃N₄.
 5. An apparatus comprising: an EUV light source collectorcomprising: a collecting mirror comprising a normal angle of incidencemulti-layer reflecting coating; an inter-diffusion barrier layercomprising a material selected from the group comprising a carbideselected from the group ZrC and NbC, or a boride selected from the groupZrB₂ and NbB₂ or a disilicide selected from the group ZrSi₂ and NbSi₂ anitride selected from the group BN, ZrN, NbN, BN, ScN and Si₃N₄.
 6. Theapparatus of claim 1 further comprising: a capping layer comprisingruthenium.
 7. The apparatus of claim 2 further comprising: a cappinglayer comprising ruthenium.
 8. The apparatus of claim 3 furthercomprising: a capping layer comprising ruthenium.
 9. The apparatus ofclaim 4 further comprising: a capping layer comprising ruthenium. 10.The apparatus of claim 1 further comprising: the multilayer reflectingcoating comprising multiple alternating layers of an absorber and aseparator.
 11. The apparatus of claim 2 further comprising: themultilayer reflecting coating comprising multiple alternating layers ofan absorber and a separator.
 12. The apparatus of claim 3 furthercomprising: the multilayer reflecting coating comprising multiplealternating layers of an absorber and a separator.
 13. The apparatus ofclaim 4 further comprising: the multilayer reflecting coating comprisingmultiple alternating layers of an absorber and a separator.
 14. theapparatus of claim 10 further comprising: the absorber comprisingmolybdenum and the separator comprising silicon.
 15. the apparatus ofclaim 11 further comprising: the absorber comprising molybdenum and theseparator comprising silicon.
 16. the apparatus of claim 12 furthercomprising: the absorber comprising molybdenum and the separatorcomprising silicon.
 17. the apparatus of claim 13 further comprising:the absorber comprising molybdenum and the separator comprising silicon.18. The apparatus of claim 1 further comprising: a capping layercomprising molybdenum.
 19. The apparatus of claim 2 further comprising:a capping layer comprising molybdenum.
 20. The apparatus of claim 3further comprising: a capping layer comprising molybdenum.
 21. Theapparatus of claim 4 further comprising: a capping layer comprisingmolybdenum.
 22. The apparatus of claim 1 further comprising: the sputterthickness rate for sputtering of a capping layer material by materialcomprising a plasma production material is at or below a rate that willresult in the capping layer material sustaining such sputtering forgreater than a selected lifetime.
 23. The apparatus of claim 2 furthercomprising: the sputter thickness rate for sputtering of a capping layermaterial by material comprising a plasma production material is at orbelow a rate that will result in the capping layer material sustainingsuch sputtering for greater than a selected lifetime.
 24. The apparatusof claim 3 further comprising: the sputter thickness rate for sputteringof a capping layer material by material comprising a plasma productionmaterial is at or below a rate that will result in the capping layermaterial sustaining such sputtering for greater than a selectedlifetime.
 25. The apparatus of claim 4 further comprising: the sputterthickness rate for sputtering of a capping layer material by materialcomprising a plasma production material is at or below a rate that willresult in the capping layer material sustaining such sputtering forgreater than a selected lifetime.
 26. The apparatus of claim 5 furthercomprising: the sputter thickness rate for sputtering of the cappinglayer material by material comprising a plasma production material is ator below a rate that will result in the capping layer materialsustaining such sputtering for greater than a selected lifetime.
 27. Theapparatus of claim 6 further comprising: the sputter thickness rate forsputtering of the capping layer material by material comprising a plasmaproduction material is at or below a rate that will result in thecapping layer material sustaining such sputtering for greater than aselected lifetime.
 28. The apparatus of claim 7 further comprising: thesputter thickness rate for sputtering of the capping layer material bymaterial comprising a plasma production material is at or below a ratethat will result in the capping layer material sustaining suchsputtering for greater than a selected lifetime.
 29. The apparatus ofclaim 8 further comprising: the sputter thickness rate for sputtering ofthe capping layer material by material comprising a plasma productionmaterial is at or below a rate that will result in the capping layermaterial sustaining such sputtering for greater than a selectedlifetime.
 30. The apparatus of claim 18 further comprising: the sputterthickness rate for sputtering of the capping layer material by materialcomprising a plasma production material is at or below a rate that willresult in the capping layer material sustaining such sputtering forgreater than a selected lifetime.
 31. The apparatus of claim 19 furthercomprising: the sputter thickness rate for sputtering of the cappinglayer material by material comprising a plasma production material is ator below a rate that will result in the capping layer materialsustaining such sputtering for greater than a selected lifetime.
 32. Theapparatus of claim 20 further comprising: the sputter thickness rate forsputtering of a capping layer material by material comprising a plasmaproduction material is at or below a rate that will result in thecapping layer material sustaining such sputtering for greater than aselected lifetime.
 33. The apparatus of claim 21 further comprising: thesputter thickness rate for sputtering of a capping layer material bymaterial comprising a plasma production material is at or below a ratethat will result in the capping layer material sustaining suchsputtering for greater than a selected lifetime.
 34. The apparatus ofclaim 10 further comprising: the multi-layer reflecting coatingcomprising a capping layer comprising a material other than the absorbermaterial and the separator material selected to have a sputter thicknessrate that will sustain sputtering by a material comprising a plasmaproduction material at or below a rate that will result in a singlelayer of the capping layer material sustaining such sputtering forgreater than a selected time and to have more favorable properties whenexposed to ambient or operating environments than those of the absorbermaterial or the separator material.
 35. The apparatus of claim 11further comprising: the multi-layer reflecting coating comprising acapping layer comprising a material other than the absorber material andthe separator material selected to have a sputter thickness rate thatwill sustain sputtering by a material comprising a plasma productionmaterial at or below a rate that will result in a single layer of thecapping layer material sustaining such sputtering for greater than aselected time and to have more favorable properties when exposed toambient or operating environments than those of the absorber material orthe separator material.
 36. The apparatus of claim 12, furthercomprising: the multi-layer reflecting coating comprising a cappinglayer comprising a material other than the absorber material and theseparator material selected to have a sputter thickness rate that willsustain sputtering by a material comprising a plasma production materialat or below a rate that will result in a single layer of the cappinglayer material sustaining such sputtering for greater than a selectedtime and to have more favorable properties when exposed to ambient oroperating environments than those of the absorber material or theseparator material.
 37. The apparatus of claim 13, further comprising:the multi-layer reflecting coating comprising a capping layer comprisinga material other than the absorber material and the separator materialselected to have a sputter thickness rate that will sustain sputteringby a material comprising a plasma production material at or below a ratethat will result in a single layer of the capping layer materialsustaining such sputtering for greater than a selected time and to havemore favorable properties when exposed to ambient or operatingenvironments than those of the absorber material or the separatormaterial.
 38. The apparatus of claim 1 further comprising: themultilayer reflecting coating comprising multiple alternating layerscomprising a multi-layer mirror optimized for a selected nominal centerwavelength in the EUV range.
 39. The apparatus of claim 2 furthercomprising: the multilayer reflecting coating comprising multiplealternating layers comprising a multi-layer mirror optimized for aselected nominal center wavelength in the EUV range.
 40. The apparatusof claim 3 further comprising: the multilayer reflecting coatingcomprising multiple alternating layers comprising a multi-layer mirroroptimized for a selected nominal center wavelength in the EUV range. 41.The apparatus of claim 4 further comprising: the multilayer reflectingcoating comprising multiple alternating layers comprising a multi-layermirror optimized for a selected nominal center wavelength in the EUVrange.
 42. An apparatus comprising: a multi-layer reflecting coatingforming an EUV reflective surface comprising: an inter-diffusion barrierlayer comprising yttrium, scandium or strontium.
 43. An apparatuscomprising: an EUV light source collector comprising: a collectingmirror comprising a normal angle of incidence multi-layer reflectingcoating; an inter-diffusion barrier layer comprising a materialcomprising yttrium, scandium or strontium.
 44. The apparatus of claim 1further comprising: the multilayer reflecting coating comprisingmultiple alternating layers comprising MoSi₂/Si or Mo₂C/Si, with orwithout an inter-diffusion barrier layer or inter-diffusion barrierlayers.
 45. The apparatus of claim 2 further comprising: the multilayerreflecting coating comprising multiple alternating layers comprisingMoSi₂/Si or Mo₂C/Si, with or without an inter-diffusion barrier layer orinter-diffusion barrier layers.
 46. The apparatus of claim 3 furthercomprising: the multilayer reflecting coating comprising multiplealternating layers comprising MoSi₂/Si or Mo₂C/Si, with or without aninter-diffusion barrier layer or inter-diffusion barrier layers.
 47. Theapparatus of claim 4 further comprising: the multilayer reflectingcoating comprising multiple alternating layers comprising MoSi₂/Si orMo₂C/Si, with or without an inter-diffusion barrier layer orinter-diffusion barrier layers.
 48. The apparatus of claim 1 furthercomprising: the multilayer reflecting coating comprising multiplealternating layers comprising Mo/X/Si/X where X comprises aninter-diffusion barrier layer comprising C or a carbide selected fromthe group ZrC and NbC.
 49. The apparatus of claim 2 further comprising:the multilayer reflecting coating comprising multiple alternating layerscomprising Mo/X/Si/X where X comprises an inter-diffusion barrier layercomprising C or a boride selected from the group ZrB₂ and NbB₂.
 50. Theapparatus of claim 3 further comprising: the multilayer reflectingcoating comprising multiple alternating layers comprising Mo/X/Si/Xwhere X comprises an inter-diffusion barrier layer comprising C or adisilicide selected from the group ZrSi₂ and NbSi₂.
 51. The apparatus ofclaim 4 further comprising: the multilayer reflecting coating comprisingmultiple alternating layers comprising Mo/X/Si/X where X comprises aninter-diffusion barrier layer comprising C or a nitride selected fromthe group BN, ZrN, NbN, BN, ScN and Si₃N₄.